In digital medical X-ray diagnosis, the X-ray radiation time, energy and flux employed must be carefully controlled to achieve optimum quality of the images recorded.
The exposure has to be selected such that the images possess high signal-to-noise ratio, and high dynamic range without being overexposed, i.e. that the detector saturates.
One method of controlling the exposure of a sensitive two-dimensional detector array, such as a CCD, is obviously to record an image, analyze it with respect to signal strengths and contrast achieved, and then adjust the exposure, whereafter a second high-quality image is recorded. While such an approach provides for the recording of high-quality images, it nevertheless suffers from a few drawbacks. Firstly, the method is time consuming: two read-outs have to be made for each object area to be imaged with intermediate analysis and adjustments. Further, the radiation dose to the object area to be imaged is higher, since it is exposed to radiation twice.
Another method, disclosed in U.S. Re. 33,634 by Yanaki, samples the radiation passed through an object to be examined during a short portion of the total exposure time by means of a sensor and adjust exposure time and the voltage, current and focal spot size of the X-ray source so that the radiation delivered by the X-ray tube during the remainder of the exposure will produce optimum contrast between structures within the object examined and optimum darkening of a film, xerographic picture, fluoroscopic image, or other recording medium. The method accounts for variations in absorption coefficient between one object to be radiographed and the next.